Creating channels in ischemnic myocardial tissue has been demonstrated to elicit an angiogenic response in the affected tissue. Revascularization has been associated with a decline the angina symptoms of patients suffering from severe diffuse coronary artery disease, especially that which is refractory to standard therapeutic modalities (Agarwal R, et al. Transmyocardial laser revascularization: early results and 1-year follow-up. Ann Thorac Surg. 1999;67(2):432-6). Clinical studies for transmyocardial revascularization have primarily involved direct access to the epicardium of the heart, which facilitates creating the desired grid of small diameter channels throughout the surface of the heart, but increases the morbidity and mortality of the procedure.
Experimental studies demonstrate an increase in perfusion for the treated tissue. Malekan, et al studied the pathophysiology of discrete bores or channels generally normal to the endocardial surface created with a CO2 laser and a power drill (Malekan R, et al. Angiogenesis in transmyocardial laser revascularization. A nonspecific response to injury. Circulation. 1998;98:II62-5). The average channel diameter was less than 1 mm for both the CO2 laser and the power drill (Malekan, et al). Newly created microvessels at high density were detected immediately within the channels, while the creation of new vessels dropped to a negligible amount at distances greater than 5 mm from the channel center (Malekan, et al). Such studies demonstrate that several discrete channels narrowly dispersed (e.g. separated by less than approximately 5 mm) throughout the chamber of the heart are needed to maximize revascularization. Therefore, to transform surgical transmyocardial revascularization into a viable catheter-based approach, the creation of channels must be optimized through remote manipulation of the cutting mechanism.
Conventional catheter-based approaches for myocardial revascularization use laser, radiofrequency, and drilling to create several discrete, small diameter bores into the myocardium. These approaches require manipulating the distal tip of the catheter to position the cutting mechanism against the endocardium, then forming bores through the endocardial surface and into the myocardium. Manipulation of a catheter within the ventricle is burdened by the trabeculated surface of the ventricles and the prominence of anatomic structures such as the papillary muscles. The ventricular anatomy therefore hinders the ability to reliably position the distal tip of a catheter at numerous unique locations along the endocardial surface. Hence, current catheter-based transmyocardial revascularization approaches do not produce an optimal dispersion of channels or bores required to maximize the therapeutic response.
Prior approaches for transmyocardial revascularization involve positioning the distal tip of a cutting element against the endocardial surface (or epicardial surface) and creating a small diameter (<1 mm) channel or bore 4 extending into the myocardium, as shown in FIG. 1b. 
As seen from FIG. 1a, which like FIG. 1b is a cut-away view that shows a thickness 9 of the myocardium, channels 4 are perpendicular to endocardial surface 2, and extend lengthwise in to the myocardial thickness. Viewed along the endocardial surface, bores 4 appear circular, and are depicted as dots in the figure. Cutting elements for prior approaches have included fiberoptics focusing laser energy into adjacent tissue, mechanical drill bits designed to bore through the myocardium, and small diameter (needle) electrodes designed to coagulate tissue while being advanced into the myocardium. As shown in FIG. 1a, prior percutaneous transmyocardial revascularization (PTMR) approaches require creating numerous channels 4 throughout the endocardial surface 2 of the heart. To produce the desired angiogenic response with prior approaches, several discrete channels 4 must be created along the endocardial surface. To maximize the angiogenic response using prior approaches, the channels need to be separated by less than about 5 mm. This maximizes the formation of new vessels throughout tissue located between discrete channels. Surgical approaches for transmyocardial revascularization can produce the desired dispersion of channels since the operator has direct access to the endocardial surface. However, it is difficult to remotely operate a catheter during percutaneous transmyocardial revascularization to accurately position the catheter to create a suitable dispersion of channels to maximize the angiogenic response.
To position prior PTMR catheters 5, physicians use preshaped introducing sheaths to direct the distal tip of a PTMR catheter 5 against the endocardial surface 2. Such introducing sheaths 6 have preformed curves configured to orient the distal tip of a prior PTMR catheter 5 against the endocardium, sufficient rigidity to stabilize the catheter against the endocardium while creating the bore, and substantial torque response to facilitate repositioning the catheter at numerous endocardial locations. Alternatively, prior PTMR catheters incorporate steering mechanisms (unidirectional or bidirectional) to help deflect the distal tip of the PTMR catheter towards the endocardial surface. Even with the aid of introducing sheaths or steering mechanisms to help position prior PTMR catheters, deficiencies associated with reliably creating a grid of channels through the endocardial surface limit the utility and effectiveness of prior PTMR catheters.
The right and left ventricles of the heart are highly trabeculated and incorporate anatomic structures, such as the papillary muscles 3, that direct the distal tips of prior PTMR catheters 5 to a limited number of unique positions. Even with introducing sheaths 6 or steering mechanisms to aid positioning prior PTMR catheters 5, the distal tips of these devices still migrate toward few unique locations while being manipulated inside the heart. The distal tips of prior PTMR catheters migrate between the trabecula of the heart and towards the base of the papillary muscles, even when a portion of the heart is infarcted. The best analogy to the problem of current methods of PTMR is cardiac ablation for ventricular tachycardia. Numerous preclinical and clinical studies involving the creation of thermally induced lesions along the endocardial surface have been performed. Common observations arose for the studies—the distal tips of steerable and preshaped catheters migrate toward six to twelve unique positions. This prevents the required distribution of channels throughout the endocardial surface to optimize the myocardial revascularization response, especially when considering that surgical transmyocardial revascularization procedures are performed with an average of twenty three discrete channels to realize the desired therapeutic response (Agarwal, et al). The inability to reliably distribute the channels throughout the endocardial surface significantly reduces the angiogenic response and accompanying therapeutic effects of the procedure.
Even attempts to incorporate guide members to more accurately position the distal cutting tip of the catheter do not effectively address the deficiencies of positioning a small diameter channel creating device at numerous positions throughout the endocardial surface of the heart. These prior devices still create channels having a circular profile or a cross-section in which the length is approximately equal to the width and a depth that is substantially greater than the diameter (or length and width). For example, U.S. Pat. No. 5,910,150 entitled “Apparatus for Performing Surgery” by Saadat describes a guide mechanism in which an end-effector (distal cutting tip) is able to move. The guide enables repositioning the end-effector at discrete locations throughout the endocardial surface, depending on the amount of advancing or retracting of the end-effector. Once positioned, this tubular end-effector must be advanced from the guide mechanism and extend into the myocardium a depth that is larger than the diameter of the end-effector. Control of the end-effector emanating from the guide mechanism decreases, the more the end-effector is extended away from the guide mechanism. In addition, numerous steps of advancing (or retracting) the end-effector, deploying the end-effector to create a discrete channel, withdrawing the end-effector, and repositioning the end-effector must be performed to create a distribution of channels capable of inducing an angiogenic response. In addition, the dispersion of channels depends on how far the operator repositions the end-effector between small diameter bores, producing inconsistencies in the distance between bores that may impact the complete angiogenic response.
When advancing prior PTMR catheters through valve structures, they are typically advanced through an introducing sheath previously inserted through the valve. Alternatively, prior PTMR catheters are steered, or otherwise manipulated into a pigtail, within the aorta, before insertion through the valve. This complicates the deployment of prior PTMR catheters into the desired heart chamber.